Imaging from Space to Earth or other Space targets rely on the use of spacecraft equipped with cameras or telescopes using refractive or reflective lenses and sensor, detectors and attitude control devices such as thrusters, reaction wheels, gyroscopes to point such optical systems in the desired direction. Using more than one of such spacecrafts imaging the same target it may be possible to achieve a resolution higher than the one of each single optic. However, this requires precise knowledge and control of the relative position of the spacecraft.
Reaction wheels, gyroscopes and other “internal systems” are ineffective to control relative positions of spacecraft; the only useful systems are based on the action-reaction principle thereby requiring ejection of mass from the spacecraft to generate the needed thrust. Therefore all such thrusters require a container for the material to be expelled which is usually stored in a fluid form such as gas or liquid. Independently on the containment method and propulsion principle, the size of the container ultimately limits the maneuvering lifetime and therefore the usability of the imaging spacecrafts. A bigger container will allow for a longer lifetime for identical other parameters.
High resolution optics, in whichever part of electromagnetic spectrum, requires big focal lengths for high magnifications and large apertures to collect sufficient amount of radiation from the far away targets to impress the detection sensors.
For a given configuration and technology, a bigger optical system will allow for a higher quality image of better resolution.
It is therefore clear that propellant containers and optical systems are competing for volume available on the spacecraft when designing them for the best performances. Having such systems separately arranged on a spacecraft requires strong structures for both of them; in fact the propellant container has to withstand pressures and accelerations loads, while the optics needs high structural stiffness to guarantee precise positioning of the lenses. Such separate strong structures increase the total mass of the spacecraft and ultimately their manufacturing and launch costs.